In the first stage of hydrocarbon recovery an energy source present in a reservoir is allowed to move to a producing wells(s) where the hydrocarbon can flow or be pumped to a surface handling facility. Typically a relatively small proportion of reservoir hydrocarbon can be recovered by this means. To increase production fluids are injected down adjacent wells to force an additional amount of hydrocarbon to the surface. This is commonly known as secondary recovery. The fluids normally used are water (such as aquifer water, river water, sea water, or produced water), or gas (such as produced gas, carbon dioxide, flue gas and various others). Additionally, if the fluid encourages movement of normally immobile residual oil or other hydrocarbons, such a process is termed tertiary recovery.
A prevalent problem with secondary and tertiary recovery projects relates to the heterogeneity of the reservoir rock strata. The mobility of the injected fluid typically is different from the hydrocarbon. For instance, when the fluid is more mobile various mobility control processes are required to make the sweep of the reservoir more uniform and the consequent hydrocarbon recovery more efficient. Unfortunately such processes have limited value when high permeability zones, commonly called thief zones or streaks, exist within the reservoir rock. The injected fluid follows a low resistance route from the injection well to the production well. In such cases the injected fluid does not effectively sweep the hydrocarbon from adjacent, lower permeability zones. Further, when the produced fluid is re-used this can lead to fluid cycling through the thief zone with little resulting benefit and at great cost in terms of fuel and maintenance of the pumping system.
Numerous physical and chemical methods have been used to divert injected fluids out of the thief zones and into or near production and injection wells. When the treatment is applied to a producing well it is usually termed a water (or gas etc.) shut-off treatment. When it is applied to an injection well it is termed a profile control or conformance control treatment.
In cases where the thief zone(s) are isolated from the lower permeability adjacent zones, mechanical seals or “plugs” can be set in the well to block the entrance of the injected fluid. If the fluid enters or leaves the formation from the bottom of the well, cement can also be used to fill the well bore to above the zone of ingress.
When the completion of the well allows the injected fluid to enter both the thief and the adjacent zones, a cement squeeze is often a suitable means of isolating the watered out zone. Certain cases, however, are not amenable to such methods because communication exists between layers of the reservoir rock outside the reach of cement. Typical examples of this are when fractures or rubble zones or washed out caverns exist behind the casing. In such instances chemical gels, capable of moving through pores in reservoir rock have been applied to seal the swept out zones.
When such methods fail the only remaining alternatives are to produce the well with poor recovery rate, sidetrack the well away from the prematurely swept zone, or abandon the well. Occasionally the producing well is converted to a fluid injector to increase the field injection rate above the net hydrocarbon extraction rate and increase the pressure in the reservoir. This can lead to improved overall recovery, but it is worth noting that the injected fluid will mostly enter the thief zone at the new injector and is likely to cause similar problems in nearby wells. Further, all of these methods are expensive.
Near wellbore conformance control methods always fail when the thief zone is in widespread contact with the adjacent, hydrocarbon containing, lower permeability zones. The reason for this is that the injected fluids can bypass the treatment and re-enter the thief zone having only contacted little or none of the remaining hydrocarbon. It is commonly known amongst those skilled in the art, that such near wellbore treatments do not succeed in significantly improving recovery in reservoirs having crossflow of the injected fluids between zones.
A few processes have been developed with the aim of reducing the permeability in a substantial proportion of the thief zone and, or at a significant distance from the injection and production wells. One example of this is the Deep Diverting Gel process patented by Morgan et al (1). This has been used in the field and suffered from sensitivity to unavoidable variations in quality of the reagents, which resulted in poor propagation. The gelant mixture is a two-component formulation and it is believed that this contributed to poor propagation of the treatment into the formation.
The use of swellable cross linked superabsorbent polymer microparticles for modifying the permeability of subterranean formations is disclosed in U.S. Pat. Nos. 5,465,792 and 5,735,349. However, swelling of the superabsorbent microparticles described therein is induced by changes of the carrier fluid from hydrocarbon to aqueous or from water of high salinity to water of low salinity.
Crosslinked, expandable polymeric microparticles and their use for modifying the permeability of subterranean formations and increasing the mobilization and/or recovery rate of hydrocarbon fluids present in the formation are disclosed in U.S. Pat. Nos. 6,454,003 B1, 6,709,402 B2, 6,984,705 B2 and 7,300,973 B2 and in published U.S. Patent Application No. 2007/0204989 A1.